Why my fasting blood sugar is 100 and 2 hours after a meal is sometimes 84 , sometimes 90? im not a diabetic, just take regular test because of family history of diabetes and because i am obese?

Blood sugar values reflect a changing interaction between blood insulin levels and ingested sugars based on the metabolic activity moment to moment. It is a range that changes over minutes to hours due to the interplay of these factors. A fasting BS of 100 suggests you are pre-diabetic and should follow your doctor's guidance to avoid becoming one.